As known, surface and epitaxial micromachining techniques allow production of microstructures within a layer deposited (for example a polycrystalline silicon film), or grown (for example an epitaxial layer), on sacrificial regions, removed at the end of the manufacturing process, by wet etching.
In general, the layers subject to micromachining (deposited or grown layer), are formed at high temperatures and have completely different operative temperatures. In addition, the various regions forming the end devices have different thermal expansion coefficients. Consequently, at the operative temperatures of the microstructures, residual mechanical stresses are present; in addition, in particular when the various regions are non-uniformly doped, the stresses are not uniform (forming stress gradients); these stresses thus cause undesirable mechanical deformations of the microstructures.
Residual stresses and stress gradients give rise to various problems in the operation of the electromechanical microstructures, as described schematically hereinafter with reference to FIGS. 1-6.
In detail, FIG. 1 shows in cross-section a polycrystalline silicon bridge element 2, formed on a monocrystalline silicon substrate 3; a sacrificial oxide layer 4 extends between the bridge element 2 and the substrate 3, except two areas, where anchorage portions 5 of the bridge element 2 extend through the sacrificial oxide layer 4, and are supported directly on the substrate 3.
FIG. 2 shows the same structure 1 as in FIG. 1, in plan view.
FIGS. 3 and 4 show the structure 1, after removal of the sacrificial oxide layer 4, when the dimensions of the structure have been reduced (shown exaggerated in the figures, for better understanding), owing to the presence of residual stress; in particular, in FIG. 3, owing to the different thermal coefficients, the dimensions of the bridge element 2 are reduced (shortened) more than those of the substrate 2; here the bridge element 2 is subjected to tensile stress, and leads to a more favorable energetic configuration. In FIG. 4 on the other hand, the bridge element 2 undergoes a lesser reduction of dimensions than the substrate 2; consequently, in this condition, the bridge element 2 tends to be lengthened in comparison with the substrate 3, but, owing to the fixed anchorage portions 5, it undergoes stress of a compressive type, causing buckling deformation.
In the case of tensile stress, the mechanical resonance frequency of bridge element 2 is shifted upwards with respect to the intrinsic value (in the absence of stress); on the other hand, in the case of compressive stress, the mechanical resonance frequency of the bridge element 2 is shifted downwards.
The average residual stress thus has the effect of modifying the resilient constant of the micromechanical structures; this modification is not reproducible, and can cause mechanical collapse of the structure (in particular in the case in FIG. 4).
The stress gradient also produces deformations of the involved micromechanical structure, as well as to variations of its mechanical features. In general, different effects occur according to the specific micromechanical structure. FIG. 5 and 6 show an example of deformation caused by the stress gradient in a structure 10 comprising a projecting element 11 of polycrystalline silicon that is not uniformly doped.
In FIG. 5, the projecting element 11 is formed on a monocrystalline silicon substrate 12; a sacrificial oxide layer 13 extends between the projecting element 11 and the substrate 12, except one area, where an anchorage portion 14 of the projecting element 11 extends through the sacrificial oxide layer 13, and is supported directly on the substrate 12.
FIG. 6 shows the structure 10 in FIG. 5, after removal of the sacrificial oxide layer 13. As can be seen, the release of the residual stress gradient causes the projecting element 11 to flex. In particular, if the function linking the residual stress with the coordinate z in the projecting element is .sigma..sub.R (Z), .sigma..sub.R is the average residual stress, .GAMMA. is the gradient of deformation (strain), and E is Young's modulus, the following is obtained:
.sigma..sub.R (z)=.sigma..sub.R +.GAMMA.Ez
In addition, if the length of the projecting element 11 is L, flexure at its free end is independent from the thickness, and is: EQU H=.GAMMA.L.sup.2 /2
Consequently, a positive strain gradient .GAMMA. makes the projecting element 11 end away from the substrate 12 (upwards), whereas a negative gradient makes it bend downwards.
In case of suspended masses, the behavior is exactly the opposite, i.e., positive stress gradients cause downward flexing, and negative stress gradients give rise to upward flexing. This is particularly disadvantageous in the case, for example, of microactuators or sensors having facing electrodes, for example fixed electrodes (with a structure similar to that of the projecting element 11 in FIG. 6), and mobile electrodes (with behavior similar to that of the suspended masses). In this case, the opposite behavior of the facing electrodes causes a reduction in the facing surfaces and consequently a reduction in the driving (in microactuators) or detection ability (for sensors).
It is also known that high temperature heat treatments help in reducing the average stress and the stress gradient of the materials; however, in particular in the case of nonuniform stress, heat treatment applied to the entire wafer of semiconductor material does not eliminate local stresses, since the individual areas are integral with the remainder of the wafer, and cannot be deformed freely; in addition these treatments cannot be carried out on the individual dies, since they are often incompatible with the end electronic components, because of possible undesired displacements of the doping quantities used in forming the various conductive regions of the electronic components.